Monday, September 15, 2008

Using Focal Length in Composition

As photographers, we frequently use the zoom feature to adjust the size of the main subject without moving the camera position. In this article, I will explore other aspects of zoom, also known as focal length, in composition.

Loosely defined, focal length is the zoom setting on the lens. It is indicated by a number, measured in millimeters, indicating the distance of the lens from the sensor. Smaller numbers, meaning the lens is closer to the sensor, produce a wider field of view. Conversely, larger numbers, meaning the lens is further from the sensor, produce more magnification with a narrower field of view.

FocalLengthCompWideAngleFocalLengthCompHighMagnificationTo get an intuitive understanding of what's happening, think about two rods that are a fixed distance apart on one end and connected by a sliding ring. As you move the ring closer to the fixed end, the free ends will move apart. As you move the ring further from the fixed ends, the free ends will move closer together. By analogy, the fixed end is the sensor in the camera, the lens is the ring and the free end represents the field of view.

Besides the obvious size of the main object, there are two other effects caused at the extremes of focal length spectrum. The first happens at short focal lengths and is called foreground expansion. The second happens at long focal lengths and is background compression.

42/52 Pointing the fingerForeground expansion magnifies items close to the lens to look proportionally larger than other items in the image. This can be used to emphasize one object over another. This image is taken at a focal length of 18mm with the finger about 2 inches from the camera lens; my body and face are at arms length. With this setup, my hand is about twice the size of my face.

Background compression happens at larger focal lengths with a narrower field of view and is where perspective causes the background to pull in closer to the subject. Here are several shots at progressively longer focal lengths: 18 mm, 55 mm, 100 mm, 205 mm and 300mm. The camera position was moved back so the main subject is the same size in all the shots. With the narrower field of view and increased magnification, even though the subject is the same size, the background is significantly changed. This can be seen best by paying particular attention to the fence and trees.
18 mm55 mm100 mm205 mm300 mm

Using zoom can make things more convenient by changing the size of the main subject from a fixed position. But next time your out shooting, remember that by moving around it can also be used to change the composition and overall feel of the image.
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